Technical Field
The present disclosure relates to a power supply system and a voltage control method for a fuel cell.
Background
In a power supply system including a fuel cell, when electric power is extracted from the power supply system in response to an electric power required by a load (hereinafter, also referred to as load-required power), there is a case where the load-required power temporarily extremely decreases even during operation of the power supply system. A system including a fuel cell generally has such a property that the energy efficiency of the overall system decreases in the case where electric power generated by the fuel cell is extremely small. Therefore, as one control that is executed when the load-required power required on the power supply system is extremely small, control for stopping the power generation of the fuel cell has been executed so far. A required electric power has been output to the load from a secondary battery mounted in the power supply system together with the fuel cell.
When the power generation of the fuel cell is stopped in the state where hydrogen remains in an anode passage of the fuel cell and oxygen remains in a cathode passage of the fuel cell, the fuel cell exhibits an extremely high open circuit voltage (OCV). When the open circuit voltage of the fuel cell becomes excessively high, the potential of the electrode (cathode) of the fuel cell becomes extremely high, and elution (degradation) of a catalyst proceeds in the cathode, with the result that the power generation performance and durability of the fuel cell decrease.
After the power generation of the fuel cell is stopped, hydrogen remaining in the anode passage permeates to the cathode passage via the electrolyte membrane of the fuel cell, and the reaction by which hydrogen is oxidized on the cathode proceeds. As a result, after a while from the stop of the power generation of the fuel cell, the open circuit voltage decreases (the cathode potential decreases) because of consumption of oxygen remaining in the cathode passage. In such a case, the cathode catalyst is reduced, and elution of the cathode catalyst more easily occurs when the cathode potential has increased again thereafter. Therefore, when the load-required power becomes extremely small, it is desired to keep the voltage of the fuel cell (electrode potential) within an appropriate range.
As a method for keeping the voltage of the fuel cell within an appropriate range when the load-required power becomes extremely small, there is suggested a method of continuing minute power generation in the fuel cell even after the load-required power becomes extremely small (see, for example, Japanese Patent Application JP 2013-161571 A). As a method of continuing minute power generation, for example, there is suggested, for example, a method of stopping supply of oxygen to the fuel cell until the output voltage of the fuel cell decreases to a lower limit of the above-described predetermined range and, after the output voltage has decreased to the lower limit, oxygen is supplied to the fuel cell until the output voltage increases to an upper limit of the predetermined range.
However, if the power generation of the fuel cell is continued even after the load-required power becomes extremely small, there can occur a situation that unrequired excessive power generation is carried out only for the purpose of keeping the voltage. Electric power excessively generated in this way is allowed to be utilized after the electric power has been once charged into a secondary battery. However, a method of storing electric power generated by the fuel cell once in the secondary battery is lower in energy efficiency than the case where electric power generated by the fuel cell is directly utilized, and leads to a decrease in the energy efficiency of the overall system including the fuel cell.